Pulsed laser sources are used in a variety of applications, including material processing, optical communications and measurements. Diode-pumped solid-state lasers, for example Nd:YAG lasers, have been conventionally used in applications requiring high peak optical power, such as marking, engraving, micro-machining, cutting, and other material-processing applications. Such lasers typically rely on Q-switching and/or mode locking to generate optical pulses and therefore produce optical pulses with characteristics that are predetermined by the cavity geometry, mirror reflectivities, and the like, and cannot generally be easily varied in the field without compromising the laser performance.
An attractive alternative to the solid-state pulsed lasers are pulsed laser systems based on fiber amplifiers (FA), such as an Erbium-doped (ED) fiber amplifier (EDFA). In some applications, EDFA-based laser systems may offer certain advantages over conventional diode-pumped solid-state lasers, such as potentially lower cost, higher efficiency, and higher reliability. Furthermore, FA-based laser systems may enable new applications by providing a combination of operational parameters, such as pulse temporal format, repetition rate, energy, power, etc., that are not accessible with other currently available technologies. Such lasers, however, typically require a seed pulse source for providing, or “seeding” the fiber amplifier with seed optical pulses. In a typical prior art arrangement, a low-power laser diode that is pulsed with the desired repetition rate and pulsewidth acts as a seed for a fiber amplifier or a chain of fiber amplifiers, which increase the pulse power. In such a configuration, the pulse generation process is separated from the amplification process, with both the spectral and temporal quality of the laser output pulses depending only on the laser diode source.
Directly pulsing the laser diode current can however generate transient effects that can disadvantageously affect both the spectrum and the noise level of the seed source. In the case of Fabry-Perot (FP) laser diodes, longitudinal mode beating can lead to high frequency noise which consequently gives rise to peak power fluctuations in the pulse structure, which is a disadvantage. Distributed feedback (DFB) and distributed Bragg reflector (DBR) diode lasers typically generate a single mode in a cw regime, but exhibit frequency chirp when pulsed, so that the laser linewidth depends on the pulse duration. For seeding a high-peak-power fiber amplifier, the source linewidth should be sufficiently broad to suppress stimulated Brillouin scattering (SBS), an undesirable nonlinear process that limits the ability to generate high peak powers in the fiber, particularly for pulse durations of more than about 1 nanosecond (ns); typically a linewidth in excess of 10 GHz is desired. The frequency chirping of the modulated DFB laser does not broaden the linewidth sufficiently to suppress SBS, limiting the use of DFB lasers to pulse duration of less than about 2 ns. In principle, the laser drive current could be modulated to broaden the linewidth, but this approach would significantly complicate the laser drive electronics. Additionally, the DFB and DBR lasers can exhibit mode hopping behaviors under some pulsed drive conditions that may cause undesirable complications in applications that rely on stable-single-frequency output.
U.S. Pat. Nos. 7,443,893, 7,457,329 and U.S. Patent applications 20080181266, 20090003395, 20090086773 disclose pulsed optical sources utilizing a seed source of continuous or quasi-continuous seed light, a continuously operating fiber-optic amplifier and an optical modulator which are coupled in various configurations using an optical circulator. In these designs, the spectral properties of resulting pulses are to some extent de-coupled from the temporal characteristic of the output pulses, as they are controlled by different elements of the pulsed source. Disadvantages of such designs include their relatively high cost and complexity requiring three or more active components, a relatively high noise level compared to semiconductor amplifiers, and drawbacks in performance related to the use of conventional optical modulators, including bias-point drift, limited contrast ratio, and photorefractive damage.
Accordingly, there is a need to provide a pulsed laser source that would be free of at least some of the deficiencies indicated above, and would be capable of generating optical pulses with flexible, user-selectable temporal and spectral formats and relatively high output power. An object of the present invention is to provide such a pulsed optical source.